Beliebers did in fact go wild. "JUSTIN BROUGHT OUT LUDACRIS DURING BABY AND I'M CRYING BECAUSE I'M SO HAPPY BECAUSE IT BROUGHT BACK MEMORIES," one Twitter user posted along with a video of the performance.

Bieber has certainly grown up in the six years since the release of "Baby." During his Atlanta show, he showcased his muscles and multiple tattoos in a tank top with Tupac Shakur's face on it -- a far cry from the purple hoodie and floppy haircut he sported back then.